1. Field of the Invention
This present invention relates to apparatus for interferometrically measuring the physical properties such as thickness and refractive index of a light-transmissive test object.
2. Description of the Prior Art
A apparatus for interferometrically measuring optical phase is disclosed in U.S. Pat. No. 2,518,647 issued on Aug. 15, 1950. This U.S. Patent shows a technique wherein the wave fronts coming from both sides of a film illuminated by a source of white light are respectively directed to an interferometer comprising a tilted translucent surface and an untilted reflecting surface, so that each of the two wave fronts from the film is amplitude-split by the tilted translucent surface and the other two wave fronts split by the reflection are tilted and directed to an interference surface, while the two wave fronts split by the transmission are reflected by the untilted reflecting surface and directed to the interference surface, whereby white interference patterns are formed on the interference surface. Then the film thickness is measured from the difference in length of optical path between the positions whereat the mutually intensifying interference patterns are formed. Similar methods of measurement are known from U.S. Pat. Nos. 2,578,859 and 2,655,073. All these are of the type in which the measurement of film thickness is effected from the difference in length of optical path between the positions whereat the mutually intensifying interference patterns are formed.
With the prior art apparatus as mentioned above, the physical properties such as thickness and refractive index of a test object such as synthetic resin film, optical thin layer vapor-deposited or coated on a suitable substrate and gap or spacing between two glass plates can be measured accurately by a non-contact- and non-destructive method. However, it is difficult to simply or directly ascertain, during measurement, which portion of the two dimensional test object corresponds to the data resulted from the measurement. In other words, it is difficult to ascertain, while observing the formed interference fringe, which portions of the test object corresponds to said interference fringe. Therefore, it is also difficult to correlate the distribution of the measured physical properties in the two dimensional test object with the extensive area of the measured test object in a simple and direct manner.